Reactor fouling caused by the formation of particles and deposits of insoluble polymeric materials ("gels") during addition polymerization procedures is undesirable from both an economic and a product quality standpoint. Various approaches have been developed to reduce or eliminate the formation of such gel deposits.
The problem of gel deposits formed during addition polymerizations seems related to the type of polymers being made. The monomers used have different degrees of polarity, different boiling points and different tendencies to self-polymerize, for example, when they condense on interior surfaces of a reactor.
For example, U.S. Pat. No. 3,988,213 to Yoshida et al. addresses the problem of gel deposit formation during the distillation of vinyl compounds such as hydroxyethyl acrylate, acrylic acid and glycidyl methacrylate. Yoshida et al. teach that these monomers have a much greater tendency to self-condense than other commonly used monomers such as styrene, vinyl acetate, esters of unsaturated carboxylic acids such as methyl acrylate and methyl methacrylate, and vinyl chloride. In addition to the use of a polymerization inhibitor, Yoshida et al. suggest the use of a specific type of distillation tower that is heated. The inner walls of the tower are maintained at a temperature sufficient to prevent condensation of the vapor being distilled. As a result, Yoshida et al. teach that such monomers can be distilled with less formation of polymers from the monomers being distilled. Such polymers become gel deposits that tend to foul the distillation tower and may contaminate the desired distilled monomer product.
A similar gel formation problem occurs during the free radical polymerization of monomers to form addition polymers. We have found that an especially troublesome gel formation problem occurs during the manufacture of addition polymers containing hydroxylated alkyl acrylates and alkyl methacrylates such as hydroxyethyl methacrylate.
It is believed that one potential source of gel particles and deposits results from the formation of initially soluble polymer deposits on the reactor surfaces and associated processing equipment. These eventually lead to higher and higher molecular weight polymers that ultimately become insoluble gels. Gel formation is much less of a problem when addition polymers are made via a batch or semi-batch polymerization process because the polymer is emptied from the reactor upon completion of the polymerization. This provides an opportunity to clean the reactor and associated processing equipment such as condensers, piping and evaporators prior to charging the next batch.
For example, U.S. Pat. No. 3,764,384 to Berni, assigned to GAF Corporation, teaches a method for removing polyvinyl halide residues from processing equipment. Berni uses a solvent cleaning technique based upon contacting such residues with N-methyl-2-pyrrolidone ("NMP") to loosen, suspend or dissolve the residues and permit them to be flushed out of the polymer processing equipment. International Specialty Products of Wayne, N.J. sells a line of GAF.RTM. chemical cleaning solvents containing NMP under the trademark ShipShape.RTM. Resin Cleaner for removing resin deposits such as those from acrylic polymers.
However, gel formation is a much greater problem when polymerization processing equipment such a continuous stirred tank reactor ("CSTR") or a tube reactor is used to continuously produce an addition polymer. Examples of the production of polymers in such equipment can be seen from an examination of U.S. Pat. Nos. 4,414,370 to Hamielec et al.; 4,529,787 to Schmidt et al.; and 4,546,160 to Brand et al. These patents teach the continuous production of various types of polymers by free radical addition processes where the polymers may include hydroxylated acrylic acid or methacrylic acid esters. These patents also teach that various solvents may be used. Hamielec et al. teaches that not all solvents are desirable to produce polymers having the desired molecular weights and narrow molecular weight distribution.
Continuous processing equipment is expensive to shut down and start up in terms of lost production time. Thus, it is very desirable to minimize shutdowns for cleaning of such equipment.
Another problem we have observed occurs when different types of addition polymers are sequentially made in the same continuous processing equipment. For economic reasons, one production CSTR is sometimes used to produce a variety of addition polymers. We have observed that when acrylic acid-containing addition polymers are made in a CSTR followed by a production run of a hydroxy-functional addition polymer containing hydroxyethyl methacrylate, gel deposit formation in the CSTR is accelerated as evidenced by gel particles in the latter polymer. Without wishing to be bound by theory, it is thought that the acrylic acid-functional addition polymer is leaving adsorbed polymeric deposits in the reactor processing equipment that serve to increase the formation of gel deposits. These deposits appear to exist even though the internal reactor surfaces tend to be visually clean to the naked eye.
Prior art attempts to reduce reactor fouling in continuous polymerization processes have, among other things, focused on the type of solvent used during the polymerization. Efforts addressed at solving this problem for rather specific combinations of monomers have failed to appreciate the problems associated with the production of addition polymers containing hydroxy-functional ethylenically unsaturated monomers.
For example, U.S. Pat. No. 5,003,021 to Kasahara et al. teaches the continuous production of 40-95% of an aromatic vinyl compound such as styrene and 5-60% of a vinyl cyanide compound such as acrylonitrile. The formation of gel-like particles in the reactor is reduced by conducting the polymerization at a temperature of from 90.degree. C. to 200.degree. C. in the presence of from about 10 to 100 parts by weight of a solvent. The solvent must contain not less than 40 weight percent of an alcohol such as methyl alcohol, ethyl alcohol, propyl alcohol, butyl alcohol, amyl alcohol and hexyl alcohol with alcohols having 3 to 6 carbons atoms such as secondary-butanol being preferred.
U.S. Pat. No. 4,351,931 to Armitage teaches the continuous manufacture of random copolymers of ethylene and acrylic acid or methacrylic acid having up to about 10 weight percent of the acid monomers. The solvent systems useful for the Armitage process are those previously used for polyethylene manufacture such as benzene, chlorobenzene or tertiary-butanol. Armitage also teaches that polymerization can be conducted in the substantial absence of added solvents except for the amount needed to dissolve the initiator. Nothing is suggested concerning the production of addition polymers containing ethylenically unsaturated monomers containing free hydroxyl groups.
U.S. Pat. No. 5,057,593 to Marshall et al. teaches a continuous process to copolymerize ethylene and carbon monoxide. Optionally, certain other comonomers may be included such as alpha-beta unsaturated carboxylic acids, esters of such acids, anhydrides of such carboxylic acids and vinyl esters of saturated aliphatic acids. Acetone is used to maintain a single phase in the reactor. The acetone is introduced concurrently with the reactant monomers to significantly reduce or eliminate reactor fouling. Column 1, lines 41-51 teach that if comonomers with reactivities substantially equal to ethylene are used, such as vinyl acetate, gel particles normally will not be produced. They conclude that in such a case, the polarity of the monomers will be similar to the that of the polymer being formed and that substantially reduces the likelihood of reactor fouling. The use of a solvent containing at least 50% acetone in the Marshall et al. process is said to overcome the effects of different monomer polarities and thus, reactor fouling, when ethylene is copolymerized with carbon monoxide in the presence of certain other optional comonomers. Use of hydroxy-functional ethylenically unsaturated monomers is not suggested.
U.S. Pat. No. 5,028,674 to Hatch et al. teaches a process for the continuous copolymerization of ethylene with certain polar comonomers such as alpha-beta unsaturated carboxylic acids, esters and glycidyl esters of such acids, and anhydrides of such acids. The copolymerization process is conducted in the presence of from about 2% to about 25% methanol as a solvent, based on the total amount of material flowing through the reactor. This solvent maintains the reactant monomers and the polymer being formed in a single phase which is said to reduce or eliminate reactor fouling. Other solvents may be included such as benzene or tertiary-butanol as long as such solvents do not prevent the methanol from reducing or eliminating reactor fouling. Nothing is taught concerning the use of hydroxy-functional ethylenically unsaturated monomers.
U.S. Pat. No. 4,135,245 to Massoubre relates to organolithium-initiated anionic polymerization of unsaturated monomers such as conjugated dienes with vinyl aromatic compounds, but does discuss the problems associated with gel formation in continuous polymerization processes.
U.S. Pat. No. 4,301,266 to Muenster et al. teaches the production of low molecular weight water soluble polymers of acrylic acid or methacrylic acid. Such monomers are polymerized at 120.degree. C. to 200.degree. C. under at least 2 bars of pressure in the presence of a free radical initiator and a solvent that is either isopropanol or a mixture of isopropanol and water where the mixture contains at least 40% by weight of isopropanol. The polymers may contain up to about 10% by weight of a comonomer such as acrylic acid or methacrylic acid alkyl esters, fumaric acid, maleic acid, monoesters or diesters of fumaric acid, monoesters or diesters of maleic acid, acrylonitrile, methacrylonitrile, acrylamide and methacrylamide. Nothing is mentioned concerning the use of hydroxy-functional ethylenically unsaturated monomers.
U.S. Pat. No. 3,575,946 to Chromecek et al. teaches the production of hydroxylated acrylate polymers in solvents for use as water-swellable eye contact lenses. For example, ethylene glycol monomethacrylate and monoacrylate that is contaminated with 0.05% to 50% of the corresponding diesters is polymerized to obtain a solvent swellable polymer. The solvents used are various polar solvents such as alcohols and may even include some amounts of water. Poor solvents for the polymers such as benzene cannot be used. The monomer concentration is cited as a limiting factor on how much diester contaminant can be present before an insoluble gel will be formed. Fully soluble polymers having molecular weights ranging from several hundred to several millions can be produced. Table 4 and the accompanying text teaches that coefficients of swelling above about 2 are useful in preparing the hydroxylated polymers of Chromecek et al. That appears to preclude the use of acetone (coefficient of swelling=1.58) because "solvents having swelling coefficients smaller than 2 cause the partial formation of insoluble polymers even under the conditions most favorable to the preparation of soluble polymers . . . " column 8, lines 48-50. As will be seen below, acetone was a preferred solvent for use in the process of the present invention.
The prior art lacks a solution to the problems associated with gel formation during the preparation of addition polymers from hydroxy-functional ethylenically unsaturated monomers using a continuous polymerization process. Particularly absent from the prior art is a solution to the gel deposit formation that occurs when the production of such hydroxy-functional polymers is preceded by the preparation of addition polymers containing free carboxyl groups such as those derived from acrylic acid or methacrylic acid. We have found that the use of propylene glycol monomethyl ether acetate ("PM Acetate") results in the formation of gel particles and deposits both below the surface of the monomer reactants and polymer and above the surface at the top of the reactor and on piping associated with the reactor. The presence of these particles and deposits reduced the quality of the polymer product and increased with time, thereby necessitating relatively frequent cleaning of the continuous reactor equipment. After extensive studies of various processes to reduce gel formation during the continuous formation of addition polymers of hydroxy-functional ethylenically unsaturated monomers, we have found a process which reduces the formation of gel particles and deposits during the continuous production of such polymers.